Hitherto, a rubber composition comprising a polybutadiene having high cis-1,4 bond content, narrow molecular weight distribution and high Mooney viscosity, synthesized using nickel-containing catalyst, has been suitably used as a rubber composition for a one-piece solid golf ball, a solid core of a solid golf ball, or a solid center of a thread wound golf ball, in order to impart rebound characteristics and durability thereto. Recently, a polybutadiene having particularly narrow molecular weight distribution, synthesized using lanthanide-containing catalyst, has been used for the same application (in Japanese Patent Kokoku Publication No. 80123/1994, Japanese Patent Nos. 2644226 and 2678240, Japanese Patent Kokai Publication No. 319148/1999 and the like).
It is disclosed, for example, in Japanese Patent Kokoku Publication No. 80123/1994 that a mixture consisting of                (i) a polybutadiene containing a cis-1,4 bond of not less than 40%, having a Mooney viscosity of 70 to 100 ML1+4 (100° C.), synthesized using nickel-containing catalyst and/or cobalt-containing catalyst, and        (ii) a polybutadiene containing a cis-1,4 bond of not less than 40%, having a Mooney viscosity of 30 to 90 ML1+4 (100° C.), synthesized using a catalyst consisting of lanthanide rare earth elements-containing compound, or a polybutadiene containing a cis-1,4 bond of not less than 40%, having a Mooney viscosity of 20 to 50 ML1+4 (100° C.), synthesized using nickel-containing catalyst and/or cobalt-containing catalyst can be suitably used as a rubber composition for golf balls. When the polybutadiene (i) is used for the rubber composition, rebound characteristics of the resulting golf ball are improved. However, when the polybutadiene (ii) having high Mooney viscosity is used in combination with the polybutadiene (i), workability when producing is degraded.        
It is disclosed in Japanese Patent No. 2644226 that a polybutadiene containing a cis-1,4 bond of not less than 80% and having a Mooney viscosity of 45 to 90 ML1+4 (100° C.) and a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 4.0 to 8.0 can be suitably used for golf balls. Although the use of the polybutadiene imparts sufficient physical properties to golf balls, improvement on rebound characteristics and durability is further required.
It is disclosed in Japanese Patent No. 2678240 that a mixture of                85 to 15 phr of a nickel or cobalt-catalyzed polybutadiene having a cis-1,4 content of more than 40% and a Mooney viscosity of less than 50, and        15 to 85 phr of a lanthanide-catalyzed polybutadiene having a cis-1,4 content of more than 40% and a Mooney viscosity of less than 50can be suitably used for a golf ball. However, when the mixture of the nickel- or cobalt-catalyzed polybutadiene having low Mooney viscosity (less than 50) and the lanthanide-catalyzed polybutadiene having low Mooney viscosity (less than 50), rebound characteristics and durability of the resulting golf ball are not sufficiently obtained.        
In Japanese Patent Kokai Publication No. 319148/1999, a solid golf ball comprising a core and a cover covering the core, of which the core comprises at least one rubber layer formed from a rubber composition comprising a polybutadiene mixture, unsaturated carboxylic acid or a metal salt of unsaturated carboxylic acid, organic peroxide and inorganic filler, is disclosed. The polybutadiene mixture consists of                (a) a polybutadiene containing a cis-1,4 bond of not less than 80 molar %, having a Mooney viscosity of 50 to 69 ML1+4 (100° C.) and a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 4.0 to 8.0, synthesized using nickel-containing catalyst and/or cobalt-containing catalyst, and        (b) a polybutadiene containing a cis-1,4 bond of not less than 40%, having a Mooney viscosity of 20 to 90 ML1+4 (100° C.), synthesized using lanthanide-containing catalyst,and a weight ratio (a)/(b) is 30/70 to 90/10. However, workability when producing and rebound characteristics of the resulting golf ball are not sufficiently obtained. Therefore, it is required to further improve the performances of the golf ball.        
In Japanese Patent Kokai Publication Nos. 149502/2001, 149503/2001, 149505/2001, 149506/2001, 149507/2001 and 149508/2001, a golf ball comprising as a constituent element a vulcanized molded article of the rubber composition comprising polybutadiene synthesized using a rare earth element-containing catalyst is disclosed. The polybutadiene is obtained by adjusting 5 mass % toluene solution viscosity, a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn), Mooney viscosity and the like to specified ranges.
However, polybutadiene having high molecular weight and high Mooney viscosity generally has poor workability when producing and poor processability. Since particularly polybutadiene synthesized using lanthanide-containing catalyst has also narrow molecular weight distribution, the flow properties are poor, and it is difficult to process it. In addition, since the processability is poor, release agent or air is remained on the surface of or in the molded article after molding, which degrades the durability. If made by a conventional method, particularly by using a conventional vulcanization condition and molding condition, it is difficult to obtain a core or golf ball having hardness distribution such that the outer portion is hard and the inner portion is soft.